This proposal is to study the effects of spatial variations in membrane properties and the pattern of cell to cell coupling on the propagation of cardiac action potentials. Numerical simulations will be done by including various membrane models in the solution of cable equations which represent the intercellular current flow. Both the normal excitation sequence and the propagational changes in abnormal conditions must be related to the physical properties of the cardiac syncytium; and we propose to learn more about the basis of arrhythmias by a systematic variation in the spatial pattern of physical properties throughout the simulated strand or sheet of tissue. Preliminary work has shown how spatial distribution of cell coupling resistance can lead to unidirectioal block, and also how the discrete nature of the cells in the syncytium can produce changes in action potential shape and velocity that are not predictable from the application of continuous cable theory.